This invention relates to improved autodeposition coating systems and novel coatings for use in such systems. Autodeposition, also known as Autophoresis.sup.R or chemiphoresis, is a process for depositing an organic coating on a metal surface. The process involves the controlled release of multivalent metal ions from the metal surface which destabilizes dispersed polymer particles in the coating such that a coating builds up on the metal surface. Such systems are well known in the coating art and are described in, for example, U.S. Pat. No. 3,776,848, U.S. Pat. No. 3,791,431, and U.S. Pat. No. 4,108,817.
Autodeposition, though a relatively new procedure for coating metal surfaces, has achieved considerable importance and wide use in the coating industry due to its many advantages over conventional systems, such as electrolytic deposition. By using autodeposition, practitioners of the art can now deposit an inorganic and an organic film simultaneously with fewer operating steps in less time while utilizing less floor space than conventional pretreatment/electrocoating systems. Autodeposition also reduces the air and water pollution associated with conventional coating systems because organic solvent usage is minimized. The use of autodeposition also reduces significantly the energy usage required by certain conventional electrocoating systems. A further advantage is the sharply decreased safety risk attained by avoiding the use of electric current in the coating bath.
There have been a number of aqueous polymer dispersions used in autodeposition coatings. Among them are dispersions of styrene-butadiene, acrylonitrile-butadiene, polyethylene, acrylic copolymers, polyvinyl chloride and tetrafluoroethylene. Although coatings formulated with these latexes are adequate for some applications their use has exposed certain performance deficiencies as autodeposition coating systems have been applied to more rigorous and demanding industrial applications. Some areas where the heretofore available autodeposition coatings have compared unfavorably with some other conventionally applied coatings in certain industrial applications are salt spray or corrosion resistance, rinse resistance, gloss, heat aging resistance and gasoline resistance.
Salt spray or corrosion resistance refers to the ability of the coating to protect the metal substrate from corrosion. Rinse resistance is the ability of the freshly applied coating to withstand a spray rinse, which is often used industrially in lieu of a dip rinse, without being washed out. Gloss, related to the uniformity of the coating, enhances the appearance of the coated article. Heat aging resistance, the ability of a coating to withstand heat during periods of storage or service without significantly weakening or losing its protective qualities, is of considerable importance because in contemporary industrial practices coated metal parts are often stored in hot warehouses for long periods before further usage or assembly. Gasoline resistance is the ability of the coating to withstand organic solvent attack without softening.
A coating which can be used in autodeposition coating systems and which will improve the corrosion resistance, rinse resistance, gloss, heat aging resistance and gasoline resistance of heretofore available autodeposition coatings would be of great advantage.